Does it make sense to you that you maybe should go on some of these trips even if you don't really want to or know in advance you'll feel bored? You alone, can decide if you'd rather NOT be bored and not go, or--
--go anyway, be bored, knowing you're mostly doing this in the context of 'catching him being good' i.e., he DID
invite you to go along and you wish to show you appreciate this consideration.
Now you know that if you go along on some of these trips you'll be bored but what I try to get couples to do is plan 1-2 'minor' activities at the reunions where just the two of you will enjoy doing something together---maybe go to a show or walk through an art museum for a few hours; just one thing you share enjoyment with together. So 90% of the time he is revisiting glory days, and maybe 5-10% of the time the two of you will take a break for a few hours to do something together you'd both enjoy.
So if you stay home, so you won't be bored, I'd advise against communicating to him that you wish he hadn't gone; or wish he would have used that time to take you somewhere else. Again, this latter stuff is catching him being bad.
You can correct his behavior somewhat, over the long run, but you buy into the idea that the key idea for you right now should be to catch him being good. I'd be curious to see what happens if you do decide to shift the way you are framing getting your husband to appreciate you more, from focusing on what you are not getting right now, the anger, resentment etc., and for at least the next 2-3 months, making daily attempts to reinforce tiny signs of the more the kind of behavior you want to see i.e., catch him being good, and increasing the likelihood that he'll do more of this in the future if you do.
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